A variety of cardiac pacemakers have been developed which rely upon measured parameters to control heart rate in order to respond to the level of activity of the patient. A number of these pacemakers seek to eliminate or at least reduce the effects of extraneous or otherwise interfering signals from the desired measured parameter. For example, Deno, U.S. Pat. No. 5,507,785, discloses a rate responsive pacemaker that is sensitive to impedance changes in the heart as an indicator of cardiac stroke volume or minute volume. This pacemaker uses a biphasic test signal to reduce or eliminate common interfering signals from the measurement of the impedance. This pacemaker also includes separate detector and injector circuits so that a variety of electrode configurations may be used.
Other proposed pacemakers are directed to certain measurements that more precisely time the events of interest in the cardiac cycle. U.S. Pat. No. 5,235,976 to Spinelli describes a parameter derived from intracardiac impedance referred to as "total active time". The active time is evaluated using the intraventricular impedance technique, the active time being the length of the interval between the onset of contraction and the point where a line passing through two points on the fast filling segment of the impedance waveform reaches the impedance level corresponding to the end-diastole impedance of the preceding beat. In other words, the impedance signal from the first part of the cardiac cycle is used to derive a minimum (pacing) interval which just accommodates systolic ejection and enough diastolic filling time to support adequate cardiac pump function.
Unfortunately, this technique depends on local (in time) impedance signal characteristics so that additional humps and variations in morphology yields an estimate of total active time that is unreliable. Furthermore, the total active time as determined by Spinelli is not valid for the many cases at high pacing rate when impedance peaks occur after the subsequent pace event.
Spinelli and other techniques also require a high sampling rate to accurately determine crossing times on the impedance waveform. Such a high sampling rate is very demanding of the power source for the pacemaker and therefor reduces the length of time that the pacemaker's installed power source may effectively perform its intended functions. Thus, there remains a need for a cardiac pacemaker that effectively controls cardiac function over a range of demands but requires a much lower rate of sampling the impedance signal over an entire cardiac cycle.
Other proposed solutions are directed to impedance signal processing and certain physiologic sensor implementations. An early example of such a pacemaker is provided in U.S. Pat. No. 4,773,401 to Citak et al. Citak et al. describe a method to determine the pre-ejection period, or PEP, and use this parameter to control pacing. With a few noteworthy exceptions, the resulting parameters of such techniques have not been sufficiently reliable and robust for commercial implementations.
Several factors make the physiologic sensing of heart function by an implantable medical device difficult and thus yield less than robust results. As a result of tradeoffs between size, weight, longevity, and power, as well as mechanical and materials compatibility, the resulting signals reflective of heart function are often contaminated. Artifacts, noise, and variations from one sensor to another and from one subject to another must be artfully dealt with in robust, practical implementations.
One important example of a challenge to the art in sensing heart function is the physiologic determination of maximum pacing rate in response to an activity sensor or paroxysmal atrial tachycardia. Other examples of the difficulties of sensing heart function include tachycardia discrimination and hemodynamic tolerance assessment for ventricular tachycardias and supra-ventricular tachycardias (SVTs), as well as pacing and anti-tachycardia pacing (ATP) capture detection for autothreshold and therapy termination and success evaluation.
Thus, there remains a need for a rate responsive cardiac pacemaker that is more immune to aberrations in sensor output waveforms, including artifacts, noise, and variations from one sensor to another. Such a pacemaker should be robust, should provide robust and reliable responses to the impedance waveform, and should be capable of practical implementation. A sensor must be combined with good signal processing and parameter extraction to assist the medical device to select appropriate therapeutic stimulation. Such a sensor should also be capable of a full range of other therapeutic and analytical functions.